1. Field of the Invention:
The present invention relates to an apparatus for manufacturing a single crystal of silicon under application of a magnetic field. Particularly, the invention relates to an apparatus suitable for manufacturing a single crystal of silicon having a large diameter of 8 inches or more.
2. Description of the Related Art:
Single crystals of silicon used as a material for semiconductor device substrates are manufactured mainly by the Czochralski method (hereinafter referred to simply as the CZ method). Recently there has also been known a so-called MCZ method (magnetic field applied CZ method) as an improvement over the CZ method. According to the MCZ method, a single crystal of silicon is manufactured while a magnetic field is applied to silicon melt contained in a crucible in order to suppress natural convection of the silicon melt induced by heating of the melt.
Recently, single crystals of silicon as large as 8 inches or more in diameter have been demanded. In manufacturing silicon single crystals of such a large diameter, a large crucible is used, which is filled with a large amount of silicon melt. For example, 100 kg or more of silicon material is charged into the crucible to be melted therein, and a single crystal of silicon is then pulled. However, when such a large amount of a silicon melt is used, natural convection induced by heating becomes vigorous, causing the silicon melt to become more and more unstable and making it difficult to grow a single crystal of silicon. Adopting the MCZ method, which can suppress such natural convection of the silicon melt, is an effective way to avoid this difficulty.
In a silicon single crystal manufacturing apparatus used in the CZ method or the MCZ method, there usually is provided a cylindrical heater surrounding the outer periphery of a crucible for heating silicon melt contained in the crucible. This cylindrical heater is formed with alternating grooves extending downward from the upper e nd and grooves extending upward from the lower end. An electric current flows vertically through each segment defined by adjacent grooves.
In a silicon single crystal manufacturing apparatus used in the MCZ method there also is provided a magnet which usually applies a horizontal magnetic field to silicon melt contained in the crucible.
Consequently, in the silicon single crystal manufacturing apparatus used in the MCZ method, a force is exerted on each segment of the heater from interaction of the electric current flowing vertically through the segment and the horizontal magnetic field applied to the silicon melt. However, since the direction of that force varies depending on the direction of the electric current flowing through the segment, great distortion and breakage of the heater may result.
Particularly, for manufacturing silicon single crystals of such a large diameter as 8 inches or more, especially 10 to 16 inches which have recently been demanded, it is necessary to use a crucible of a large diameter as mentioned above and further necessary to use a heater which is also of a large diameter to match the crucible.
Into the crucible of a large diameter is then charged a large amount of silicon melt. For maintaining such a large amount of silicon melt at a constant temperature during growth of a single crystal of silicon, it is necessary to increase the calorific value of the heater. Consequently, the working electric power increases, resulting in an increase of the electric current flowing through the heater.
On the other hand, for suppressing the vigorous natural convection of such large amounts of silicon melt occurring within crucibles of such a large diameter, it is necessary to apply a magnetic field of a high intensity to the silicon melt.
For the growth of a silicon single crystal with a diameter as large as 8 inches or more it is necessary to supply a large electric current and a high intensity magnetic field, making the heater distortion more conspicuous and increasing the likelihood that distortion will cause breakage of the heater.
Further, the heater used in the manufacture of a silicon single crystal as large as 8 inches or more in diameter is itself very heavy causing distortion of the heater due to its own weight to also pose a problem.
Under these circumstances, development of an apparatus capable of manufacturing a single crystal of silicon without causing breakage of the heater used, even if a large electric current flows through the heater, even if a magnetic field of a high intensity is applied to the silicon melt in the crucible, and even if the heater is of a larger diameter, has been desired.